(1) Field of the Invention
The invention relates to the operational design of hard disk drives and more particularly, to an improved read/write seek procedure and associated components which provide good access times as well as reduced acoustic noise.
(2) Background of the Invention and Description of Previous Art
A disk drive is a data storage device that stores digital data onto concentric circular tracks defined on the surface of a data storage disk. Data is read from or written to a track of the disk using a transducer mounted on the tip of a head assembly of an actuator arm. The transducer comprises a read/write element. The element is held close to a desired track while the disk spins about its center at a substantially constant angular velocity. To properly locate the transducer near the desired track during a read or write operation, a closed-loop servo scheme is generally implemented to move the actuator arm. The servo scheme uses servo data read from the disk surface to align the transducer with the desired track.
In order to move the read/write head from over one track to over another, a controller sends a sequence of signals to a voice coil mounted within the field of a permanent magnet at the distal end of the actuator arm. The voice coil moves within the magnetic field according to the strength of the signal and thereby positions the read/write head at the other end of the actuator arm from one position to another over the tracks of the hard disk. The signals are derived from an procedure which is programmed into the controller and which optimizes the speed of the head for a smooth error free head travel. The procedure controls the speed of the read/write head from its starting location over an arbitrary track on the storage disk to its final position over a target track. The starting location is either over the track whereon the previous read/write operation was performed or at some other arbitrary or “parking” location. The path taken by the head is over an arc determined by the locus of the actuator arm to which the head is attached. Those skilled in the art are familiar with the operation of head controllers.
In order to move the head from its initial position to its target location as quickly as possible, the servo system must have instantaneous knowledge of the heads position, velocity, and acceleration and from this information, compute and dispatch incrementally forward looking values for these parameters to the actuator. This is accomplished by a feedforward control which causes the head to respond accordingly. The feedforward control is able to minimize tracking errors caused by known disturbances by compensating for them in advance. The process of directing the head from its starting location to its target location is referred to as a “seek” operation and the path by which the head travels is defined by a seek procedure.
Chu, et. Al., U.S. Pat. Nos. 6,801,384 B2 and 6,744,590 B2 describe the communication between a controller and a magnetic read/write transducer in detail wherein the path of the transducer is controlled by a sinusoidal seek procedure. In '590 the seek time is adjusted by observing the maximum velocity error of the previous seek. '384 includes all actuator electrical characteristics in setting the acceleration level. It is common practice to account for back emf in ‘bang-bang’ seeks. Both show a sine wave seek where position, velocity and acceleration are individually controlled in the time domain. Kobayashi, et. Al., U.S. Pat. No. 6,153,997 cites a seek procedure where the acceleration trajectory follows a trapezoidal/exponential function
Squires, et. al., U.S. Pat. No. 6,279,108 B1 cites a long seek procedure which is typical of those presently in use. This method produces good access times but also unwanted acoustic noise. It would be desirable to significantly reduce the acoustic noise while still obtaining good access times. Burton, et. Al., U.S. Pat. No. 6,515,820 describes a sine seek procedure whereby the velocity of the seek starts at zero and accelerates, following a sine wave until an acceleration saturation occurs. The sine seek controller receives velocity information and dispatches velocity feedforward data. It also computes a position error and a switch point curve. The switch point curve is is computed as an offset to a demand velocity curve which is stored in memory. When the velocity crosses the switch point curve, the feedforward signal changes from acceleration to deceleration and the velocity decreases until the demand velocity curve is crossed and then decreases linearly, reaching zero when the desired read/write head is over the target track.
Generally, a seek procedure independent of time is superior, particularly when gain variations and/or nonlinearities are present. This is so because It is found that forcing acceleration, velocity and position to zero simultaneously at a specific time is more difficult than forcing these parameters to zero without a time constraint.
If a seek is off the prescribed trajectory, as it typically is, the loop self-corrects by adjusting the acceleration and driving the seek back onto the trajectory, independent of time. If the acceleration feedforward is too low, the seek just takes a little longer but acceleration, velocity, and PES (Position Error Signal) are all simultaneously driven toward zero.